Until now, our knowledge of pregnancy in the world’s largest living fish came from one female
BY REASEARCHERS;
How do you know if the world’s largest living fish is expecting babies? Not by her bulging belly, it turns out.
Scientists thought that an enlarged area on the undersides of female whale sharks was a sign of pregnancy. But a technique used for the first time on free-swimming animals showed only skin and muscle. These humps might instead be a secondary sex characteristic on mature females, like breasts on humans, researchers report in the March 23 Endangered Species Research.
The ultrasound is part of a suite of new methods including underwater “jet packs” and blood tests that scientists hope could unlock secrets about this creature’s reproduction.
Whale sharks (Rhincodon typus) are classified as globally endangered by the International Union for Conservation of Nature. There are only an estimated 100,000 to 238,000 individuals left worldwide, which is more than a 50 percent decline in the last 75 years.
In part because whale sharks are relatively rare, their reproductive biology is mostly a mystery . Nearly everything biologists think they know is based on the examination of one pregnant female caught by a commercial fishing boat in 1995.
“Protecting organisms without knowing about their biology is like trying to catch a fly with our eyes closed,” says Rui Matsumoto, a fisheries biologist with the Okinawa Churashima Foundation in Japan. The organization researches subtropical animals and plants to maintain or improve natural resources in national parks.
To learn more about these gentle giants, Matsumoto and shark biologist Kiyomi Murakumo of Japan’s Okinawa Churaumi Aquarium had to figure out how to keep up with them. Like superheroes in a comic book, the biologists used underwater jet packs — propellers attached to their scuba tanks — to swim alongside the fish, which average 12 meters in length and move about five kilometers per hour.
Then the researchers had to maneuver a 17-kilogram briefcase containing a waterproof ultrasound wand on the undersides of 22 females swimming near the Galápagos Islands and draw blood with syringes from their fins. Until this study, the ultrasound wand had never been used outside of an aquarium on free-swimming wildlife.
Performing these two tests on whale sharks is especially challenging, says study coauthor Simon Pierce, a whale shark ecologist with the Marine Megafauna Foundation, a nonprofit organization that uses research to drive marine conservation. The fish “have some of the thickest skin of any animal — up to about 30 centimeters thick.”
Another challenge is the seawater itself, which can contaminate blood samples. The researchers developed a two-syringe system, where the first syringe creates a vacuum and allows the second syringe to draw only blood.
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